1. Field of the Invention
The present invention relates to a fixing device of an electromagnetic induction heat system for heating a fixing roller, and more particularly, to a technology for preventing excessive heating of the vicinities of the edges of said fixing roller.
2. Description of the Related Art
Art A fixing device of an electromagnetic induction heat system for heating a fixing roller by electromagnetic induction is used in image forming apparatuses, such as printers, copiers, facsimiles, and MFPs thereof.
More specifically, an electromagnetic coil is disposed inside of a tubular fixing roller made of magnetic material, and alternating current is provided therein so that flux is generated. This generates eddy current (induced current) to produce heat in the fixing roller, resulting from electromagnetic induction occurred by flux (magnetic field) in the electromagnetic coil. The image forming apparatus structured as such generally conducts temperature control of the fixing roller based on the detection result of the temperature sensor, that detects the temperature of the fixing roller, by means of controlling alternating current applied to the electromagnetic coil.
When a small-sized paper is used in printing output, there have been conventionally occurred a problem of excessive heating of a non-paper-passing area, where the paper does not pass through, in the vicinity of the edge of the fixing roller. Here, for example, Japanese Unexamined Patent Publication No. 2004-325678 has been disclosing a structure which uses magnetic shunt alloy in the fixing roller for the purpose of preventing excessive heating in the non-paper-passing area in the fixing roller. For confirmation, magnetic shunt alloy has a property for rapidly decreasing its magnetic quality when it reaches the Curie point.
In such a structure, the magnetic quality in the non-paper-passing area rapidly decreases when the temperature in the non-paper-passing area of the fixing roller exceeds the Curie point of its magnetic shunt alloy, thereby rapidly reducing the eddy current occurred from electromagnetic induction. Hence, excessive heating of the non-paper-passing area of the fixing roller can be prevented without using a temperature sensor, a thermostat, and the like.
On the other hand, for example, Japanese Unexamined Patent Publication No. 2003-223063 has been disclosing a structure in which a metal, such as copper, having diamagnetism is provided in the circumference of the fixing roller made of magnetic materials. For confirmation, diamagnetism means a nature to be magnetized in the opposing direction of the flux. In such a structure, when the flux passing through the fixing roller from the electromagnetic coil leaks, this leaked flux can therefore be prevented to leak to the outside because of the counteraction of the metal of diamagnetism.
However, both Japanese Unexamined Patent Publications No. 2004-325678 and No. 2003-223063 are still having a problem that the flux, which is leaked from the fixing roller due to the temperature rise in the non-paper-passing area of the fixing roller, is counteracted or discharged as it is to the outside, leaving the energy of the flux vainly consumed.
In view of the above problem residing in the prior arts, it is an object of the present invention to provide an image forming apparatus and a fixing device therefor which efficiently utilize the energy of the leaked flux that is leaked from the vicinities of the edges of the fixing roller.